1. Field of the Invention
The present invention relates to a conductive paste and a laminated ceramic electronic component made using this conductive paste to form internal conductor films. In particular, the present invention relates to an improvement advantageous for a decrease in the layer thickness and an increase in the number of layers of a laminated ceramic electronic component.
2. Description of the Related Art
Component bodies provided in laminated ceramic electronic components, for example, monolithic ceramic capacitors, have a laminated structure composed of a plurality of laminated ceramic layers with internal conductor layers extending along interfaces between the ceramic layers. In general, such a component body is manufactured by the steps of forming a film made of a conductive paste which is to become the internal conductor film on a ceramic green sheet which is to become a ceramic layer by printing, etc., laminating a plurality of ceramic green sheets including the ceramic green sheets provided with such a conductive paste film, and firing the resulting green laminate at a high temperature.
A paste used as the aforementioned conductive paste is prepared by dispersing a conductive powder in an organic vehicle composed of an organic binder and a solvent. The conductive paste for the internal conductor film conventionally contains a conductor using a powder made of a noble metal such as, for example, palladium and platinum. However, an inexpensive powder made of a base metal, for example, nickel, has come to be used as the conductive powder contained in the conductive paste in order to reduce the product""s cost.
On the other hand, requirements for miniaturization and an increase in the number of layers of laminated ceramic electronic components have become intensified in the market. In particular, the requirements for miniaturization and an increase in the capacitance of monolithic ceramic capacitors have become intensified. Consequently, a decrease in the layer thickness of the ceramic layers has accelerated, and accompanying this decrease in the layer thickness of the ceramic layer, a decrease in the layer thickness of the internal conductor films has also accelerated. In order to decrease the layer thickness of the internal conductor film, it is effective to decrease the particle diameter of the conductive powder contained in a conductive paste.
When a nickel powder, for example, is used as the conductive powder, the nickel itself is likely to oxidize at a high temperature. Consequently, the firing step is performed in a nonoxidative atmosphere, for example, an inert atmosphere and a reducing atmosphere. Since the oxidization velocity of nickel significantly depends on the specific surface area of the nickel powder, the nickel becomes likely to be oxidized with a decrease in the particle diameter of the nickel powder, and concomitant surface area increase, even when the firing is performed in the nonoxidative atmosphere. Therefore, structural defects due to oxidization of nickel are still likely to occur.
Furthermore, accompanying the decrease in the particle diameter of the nickel powder, sintering and shrinkage of the nickel powder start at a relatively early stage of the firing during the firing step. Consequently, the difference of the shrinkage starting temperature and the quantity of shrinkage between the ceramic layer and the conductive paste film are increased when a green laminate is integrally fired. As a result, a relatively large stress is generated inside the laminate, and thereby, structural defects, for example, delamination and cracking, become likely to occur.
In order to overcome the aforementioned problems, a ceramic material powder having the same or nearly the same composition as that of a ceramic material powder contained in the ceramic layer can be added to the conductive paste for the sake of suppressing or controlling the sintering shrinkage of the conductive paste. It is believed that the aforementioned ceramic material powder added to the conductive paste stays between nickel particles in the conductive paste, acting as a pinning material during the sintering of the green laminate, and thereby, performs a function of suppressing the sintering shrinkage of the conductive paste film which is to become an internal conductor film.
However, when a fine-particle nickel powder having an average particle diameter of about 0.2 xcexcm or less, for example, is used in the conductive paste in order to further decrease the layer thickness of the internal conductor film in the laminated ceramic electronic component, it becomes difficult to achieve an adequate effect of suppressing the sintering by only performing addition of the aforementioned ceramic material powder since the specific surface area of the nickel powder itself is further increased, and the frequency of contact of the nickel particles with each other is further increased, in the conductive paste with a decrease in the particle diameter.
Furthermore, spheroidizing due to sintering of a nickel powder becomes more likely to occur with a decrease in the particle diameter of the nickel powder. This spheroidizing of nickel interferes the continuity of the internal conductor film, and causes a reduction of the coverage of an internal electrode. The spheroidizing of nickel can be prevented to some degree by the addition of the aforementioned ceramic material powder. However, when the particle diameter of the nickel powder is further decreased, it becomes difficult to completely prevent the spheroidizing of nickel. Consequently, the continuity of the internal conductor film is impaired with a decrease in the layer thickness of the internal conductor film, and its coverage is thereby reduced. As a result, the design capacitance of the monolithic ceramic capacitor may not be achieved.
Although the nickel powder was primarily described above, substantially similar problems may be encountered in the case of a powder of silver, silver-palladium alloy, copper, etc.
Accordingly, it is an object of the present invention to provide a conductive paste capable of overcoming the aforementioned problems and a laminated ceramic electronic component constituted using this conductive paste.
A conductive paste according to an aspect of the present invention contains a conductive powder and an organic vehicle, and further contains an organic acid barium salt and an organic zirconium compound in order to overcome the aforementioned technical problems. The content of each of the organic acid barium salt in terms of barium atom and the organic zirconium compound in terms of zirconium atom is about 0.05 to 1.00 mol relative to one mol of the conductive powder, and the content of the organic zirconium compound in terms of zirconium atom is 0.98 to 1.02 mol relative to one mol of the organic acid barium salt in terms of barium atom.
Preferably, a nickel powder is used as the aforementioned conductive powder.
The conductive paste according to the present invention is in particular applied with advantage when the average particle diameter of the conductive powder contained therein is about 0.2 xcexcm or less.
The conductive paste according to the present invention is used with advantage for forming internal conductor films extending along interfaces between a plurality of laminated ceramic layers.
According to another aspect of the present invention, a laminated ceramic electronic component including a plurality of laminated ceramic layers and internal conductor films extending along interfaces between the ceramic layers is provided. In the laminated ceramic electronic component according to the present invention, the aforementioned internal conductor films are products of firing of the conductive paste according to the present invention.
Preferably, the aforementioned ceramic layer is composed of barium titanate.
The laminated ceramic electronic component according to the present invention is applied to a monolithic ceramic capacitor with further advantage. In this case, the internal conductor films are arranged in order to achieve a predetermined capacitance via the ceramic layers and, in addition, the laminated ceramic electronic component is provided with external electrodes arranged on outer surfaces of a laminate composed of a plurality of the ceramic layers and electrically connected to predetermined internal conductor films for using the capacitance.
As described above, the conductive paste according to the present invention contains the conductive powder, the organic vehicle, as well as the organic acid barium salt and the organic zirconium compound, the content of each of the organic acid barium salt in terms of barium atom and the content of the organic zirconium compound in terms of zirconium atom is specified to be about 0.05 to one mol relative to one mol of the conductive powder, and the content of the organic zirconium compound in terms of zirconium atom is specified to be about 0.98 to 1.02 mol relative to 1.00 mol of the organic acid barium salt in terms of barium atom. Consequently, even when the average particle diameter of the conductive powder is reduced to, for example, about 0.2 xcexcm or less, the sintering thereof can be achieved without adversely affecting the electrical characteristics of the laminated ceramic electronic component using the aforementioned conductive paste, for example, without changing the capacitance change rate with temperature of the monolithic ceramic capacitor.
Therefore, a decrease in the layer thickness and high coverage with the internal conductor film formed using this conductive paste can be simultaneously achieved, and structural defects of the laminated ceramic electronic component constituted using this conductive paste can become unlikely to occur.
Consequently, when the conductive paste according to the present invention is in particular used for forming the internal conductor film provided in the monolithic ceramic capacitor, miniaturization and an increase in the capacitance of the monolithic ceramic capacitor can be achieved with advantage while the high reliability is maintained.